Methods and apparatus for imaging in conjunction with radiotherapy

ABSTRACT

Imaging may be performed using a megavoltage (MV) radiotherapy treatment system. An electron beam directed at a low-Z target generates an imaging cone beam. The cone beam may be shaped to conform to projections of volumes of interest in a subject. Image filling may be performed to reduce artifacts in the volumes of interest. Image data for filling may be derived from digitally reconstructed radiographs.

TECHNICAL FIELD

The invention relates to medical imaging and to radiotherapy.Embodiments provide imaging methods that may be performed using MeVradiation sources.

BACKGROUND

Cancer is a disease characterized by the rapid uncontrolled growth ofcells that are able to invade nearby tissues, as well as metastasize toother areas of the body. Several methods for cancer treatment areemployed today. These include systemic treatments such as chemotherapy,hormone therapy or biological therapy, and local treatments such assurgery, cryosurgery, or radiotherapy.

Radiotherapy is an important treatment for many types of cancer. Recentadvances in radiotherapy have provided finer control over thedistribution of radiation dose delivered to subjects' tissues. This finecontrol can be exploited to permit radiation to be delivered to a lesionsuch as a tumor while sparing normal tissue that is closely adjacent tothe tumor. Imaging is an important adjunct to radiotherapy. Imaging isused to identify the extent of lesions that may be treated byradiotherapy as well as to determine the location of the lesion relativeto other nearby anatomical structures. Imaging is also used to monitorthe response of a subject to treatment.

The use of radiotherapy is not limited to cancer treatment. Radiotherapycan also be useful in the treatment of other conditions.

Image-guided radiation therapy (IGRT) is a technique that involvesacquiring images during a course of radiation therapy. IGRT can deliverradiation with improved accuracy by taking into account changes in thesubject as revealed by the images. Images may be taken before or duringthe delivery of radiation. Some radiation sources, such as linearaccelerators are equipped with imaging systems such as kV X-ray imagersfor acquiring images of a subject while the subject is positioned forthe delivery of radiation.

SUMMARY OF THE INVENTION

The invention has a number of aspects. Some of these aspects relate tofeatures that can be applied individually or in combination with otheraspects. A non-limiting list of aspects of the invention includes:treatment planning systems for planning radiotherapy treatments thatinclude functionality for planning imaging sequences; methods forplanning radiotherapy treatments that take into account imaging dose;methods for acquiring images in the course of radiotherapy treatments;radiotherapy treatment systems that incorporate imaging functionality;and media containing computer instructions for causing a processor toperform methods as described herein.

One aspect provides methods for imaging comprising generating a coneX-ray beam by directing a megavolt electron beam at a low-atomic-numbertarget, shaping the cone X-ray beam to match a shape of a volume ofinterest (VOI) in a subject, and detecting X-rays of the cone X-ray beamthat have passed through the subject at an imaging X-ray detector. Themethod may be practised using a medical linear accelerator to generatethe electron beam. In some embodiments, shaping the cone X-ray beamcomprises adjusting positions of leaves of a multi-leaf collimatorand/or rotating a multi-leaf collimator about its axis.

In some embodiments the low-atomic-number target is supported on agantry that is rotatable relative to the subject and the methodcomprises repeating: shaping the cone X-ray beam to match a shape of avolume of interest in a subject; and detecting at the imaging X-raydetector X-rays of the cone X-ray beam that have passed through thesubject to obtain a plurality of images for a corresponding plurality ofdifferent angles of the gantry relative to the subject.

Another aspect provides a method for planning a radiation treatment fordelivery by a radiotherapy apparatus comprising a radiation source thatis rotatable to different beam angles around a subject and a beam shaperconfigured to control a shape of a radiation beam emitted by theradiation source. The method comprises defining at least one set ofimaging conditions. Each set of imaging conditions comprises at least abeam angle and a beam shape for exposing at least one volume of interestto radiation. The method estimates a volumetric radiation dose for theat least one set of imaging conditions and establishes a plan for atherapeutic radiation treatment. The plan comprises apertures for aplurality of beam angles. Establishing the plan comprises optimizing theapertures to deliver a desired radiation dose to a target region of asubject while maintaining radiation dose to tissues outside of thetarget region below one or more thresholds. Establishing the plancomprises taking into account the estimated volumetric radiation dosefor the at least one set of imaging conditions at least in a selectedregion outside of the target region.

In some embodiments, optimizing the apertures comprises estimatingvolumetric radiation doses for the apertures and summing the volumetricradiation doses for the apertures together with the estimated volumetricradiation dose for the at least one set of imaging conditions.

In some embodiments the selected region outside of the target regioncorresponds to a sensitive tissue desired to be spared by the radiationtreatment and the optimization comprises applying a cost function thatvalues minimizing dose to the selected region.

Another aspect provides a method for planning a radiation treatment. Themethod comprises: planning exposures of a subject to radiation to beused for imaging; computing a contribution to dose from the imagingexposures; and using the imaging dose contributions in generating atreatment plan. The method may be performed automatically by acomputerized treatment planning system. The treatment plan may comprisecontrol signals that may be applied to control a radiation deliverysystem.

Another aspect provides an imaging method comprising, for each of aplurality of different beam angles, controlling a beam shaper to shape aradiation beam such that delivery of radiation is primarily limited topaths that pass through a plurality of volumes of interest within asubject; obtaining images of radiation that has passed through thevolumes of interest; and, processing the images to obtain volumetricimages of the plurality of volumes of interest.

Further aspects of the invention and features of specific exampleembodiments of the invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate non-limiting embodiments of theinvention.

FIG. 1 is a block diagram of radiotherapy apparatus according to anexample embodiment.

FIG. 2 is a graph illustrating CNR as a function of dose for bone andlung tissue images for three different X-ray beams.

FIG. 3 is a flow chart illustrating an imaging method according to oneembodiment.

FIG. 4 is a flow chart illustrating an example method for fillingimages.

FIG. 5 illustrates an example image, a digitally-reconstructedradiograph (DRR) and a filled image.

FIG. 6A is a graph showing contrast as a function of dose for CT resultsbased on filled and unfilled images.

FIG. 6B is a graph showing noise as a function of dose for CT resultsbased on filled and unfilled images.

FIG. 6C is a graph showing CNR as a function of dose for CT resultsbased on filled and unfilled images.

FIGS. 7A and 7B illustrate dose from an imaging exposure to cone-beamX-ray radiation as a function of position for 4 cm diameter and 8 cmdiameter cylindrical volumes of interest.

FIG. 8A shows a beam's-eye view of a plurality of volumes of interestprojected into the plane of a multileaf collimator.

FIG. 8B is an example of an arrangement of volumes of interest viewedfrom an angle for which it is impossible to use a multileaf collimatorto shape an X-ray beam to match the projection of the volumes ofinterest.

FIGS. 8C and 8D illustrate two different configurations of the leaves ofa multileaf collimator.

FIG. 9 is a flow chart illustrating a method for obtaining 3D image datacovering multiple volumes of interest.

FIG. 10 is a block diagram of a radiotherapy system that includes atreatment planning unit operating in conjunction with a radiationdelivery machine.

FIG. 11 illustrates one method for using a 3D imaging dose distributionin the optimization of a treatment plan.

DESCRIPTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements have not been shown or described indetail to avoid unnecessarily obscuring the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative, ratherthan a restrictive, sense.

FIG. 1 is a block diagram of radiotherapy apparatus 10. Radiotherapyapparatus 10 comprises a source 12 of radiation. In the illustratedembodiment, source 12 comprises a linear accelerator 14 that acceleratesa beam 15 of electrons to MeV energies and directs the electron beam 15at a target 16 of a high atomic number (high Z) material. Target 16 may,for example, comprise tungsten or tungsten backed with copper. Suitabletargets for generating X-rays suitable for radiotherapy in a linearaccelerator are commercially available. Electron beam 15 interacts withtarget 16 to generate a beam 17 of X-rays.

X-ray beam 17 is shaped by upper and lower sets of jaws 18A and 18B anda multileaf collimator 20 before being delivered toward a patientsupport 19 on which a subject may be supported for treatment. Apparatus10 additionally includes a flattening filter 22 that increases theuniformity of the fluence of X-ray beam 17 and an ionization chamber 23that can be applied to measure the fluence of X-ray beam 17.

Apparatus 10 comprises a gantry 25 that permits an angle θ at whichX-ray beam 17 is incident toward patient support 19 to be rotated. Insome cases, gantry 25 permits rotation through a full 360 degrees aroundpatient support 19.

An imaging detector 30 is opposed to target 16 such that some X-rays inbeam 17 originating at target 16 can pass through a subject on patientsupport 19 and be detected by imaging detector 30. Imaging detector 30may comprise, for example, an electronic portal imaging device.

Imaging detector 30 may comprise an amorphous silicon flat paneldetector. Such detectors are in widespread use for detecting MV photonsin medical linear accelerators. Such detectors typically comprise alayer of copper overlying the active detector matrix. The copper layerincreases detection efficiency for MV photons. Where imaging isperformed with lower energy photons, as described below, it isadvantageous (but not required) that imaging detector 30 not have such acopper layer.

A treatment planning system 40 generates control parameters forapparatus 10. The control parameters may, for example, specify a numberof units of radiation to be delivered to a subject for each of somenumber of corresponding gantry angles, MLC rotation angles and MLC leafsettings. The control parameters may specify conditions for a number ofdiscrete radiation exposures (a step-and-shoot mode) and/or conditionsfor dynamic delivery of radiation (e.g. delivery of radiation while aconfiguration of apparatus 10 is changing). Apparatus 10 may becontrolled according to the control parameters generated by treatmentsystem 40 to deliver radiation to a subject according to a treatmentplan.

Treatment planning system 40 may comprise a computer system executingsoftware that generates a treatment plan under the supervision of and/orwith the assistance of a human operator. Treatment planning system 40has access to a set of image data for a subject. The image data may, forexample, comprise 3D data such as results of a computed tomography (CT)scan. In the illustrated embodiment, treatment planning system 40 hasaccess to a data store 42 containing imaging data 43 for a subject.

One example of a treatment planning system is the ECLIPSE™ treatmentplanning system available from Varian Medical Systems of Palo AltoCalif.

Contrast to noise ratio (CNR) is a useful indicator of image quality.One way to define CNR is as follows:

$\begin{matrix}{{C\; N\; R} = \frac{{{\overset{\_}{S}}_{m} - {\overset{\_}{S}}_{b}}}{\sigma_{b}}} & (1)\end{matrix}$

where: S _(m) is the average signal in the subject being imaged; S _(h)is the average signal in the background and σ_(b) is the standarddeviation of the signal in the background.

CNR can be increased by increasing dose because, in general, theavailable means for increasing dose (e.g. increasing the number ofexposures by taking images from more angles, increasing the X-rayfluence rate, or increasing slice size) can all result in reductions inquantum noise. However, it is generally considered desirable to keepimaging doses as small as practical.

It has been determined that when X-rays resulting from the interactionof electron beam 15 with low-Z target 50 are used for imaging, theresulting images can have significantly higher contrast to noise ratio(CNR) than images based on X-rays resulting from the interaction ofelectron beam 15 with high-Z target 16.

To facilitate improved imaging, apparatus 10 includes a second target50. Second target 50 comprises a low-atomic-number (Low Z) material. Forexample, second target 50 may comprise aluminum (atomic number 13) orberyllium (atomic number 4) or another suitable element having an atomicnumber in the range of 6 to 13, for example. In some embodiments secondtarget 50 comprises an element having an atomic number in the range of 8to 20. Second target 50 is preferably sufficiently thick that electronbeam 15 does not pass through second target 50 significantly. In someembodiments, second target 50 has a thickness of 5 mm or more or 3 mm ormore. However, in some embodiments, especially those in which the energyof electron beam 15 is reduced, second target 50 may be thinner andstill stop essentially all electrons of electron beam 15. It istypically advantageous to make second target 50 no thicker thannecessary to provide appropriate mechanical strength and stop electronbeam 15.

In the illustrated embodiment, apparatus 10 comprises an actuator 52configured to insert low-Z target into electron beam 15 while removinghigh-Z target 16 from the path of electron beam 15 or vice versa. Thereare a number of ways in which switching among targets 16 and 50 may beaccomplished. These include, for example, steering electron beam 15 toone or the other of targets 16 and 50; rotating or translating acarousel or other carrier on which targets 16 and 50 are supported;providing separate mechanisms for moving targets 16 and 50 into and outof the path of electron beam 15 and controlling those mechanisms in acoordinated manner; manually operating a mechanism to remove target 16and replace it with target 50; etc.

A beneficial feature arising from the use of a low-Z target 50 forgenerating an imaging beam is that, for a given electron beam current,X-ray photon generation is less efficient that for higher-Z targets. Fortypical electron currents produced by a medical linear accelerator,precise control of radiation dose can be maintained even at lowexposures.

It has also been determined that CNR may be further improved by imagingwithout a flattening filter 22. The illustrated apparatus 10 comprisesan actuator 53 for moving flattening filter 22 into or out of the pathof X-ray beam 17. In some embodiments actuators 52 and 53 are combinedor operated in a coordinated fashion to provide an imagingconfiguration—in which electron beam 15 impinges on low-Z target 50 andflattening filter 22 is not present, and a radiotherapy configuration—inwhich electron beam 15 impinges on high-Z target 16 and flatteningfilter 22 is present in the path of X-ray beam 17.

Some linear accelerators provide carousels intended for holdingflattening filters. The carousels are rotatable to bring a desiredflattening filter into the beam. Some embodiments exploit the carouselto hold low-Z target 50 in place of a flattening filter. In suchembodiments the linear accelerator may be placed into an imaging mode byremoving high-Z target 16 from electron beam 15, rotating the carouselto bring low-Z target 50 into the path of the electron beam and settingparameters of the electron beam (e.g. beam current and beam energy) toyield an X-ray beam 17 having properties that are better for imaging(e.g. providing better contrast) than the X-ray beam 17 resulting fromimpingement of the MeV electron beam 15 on high-Z target 16.

The quality of X-ray beam 17 can be further improved by reducing theenergy of electron beam 15. For example, some medical linearaccelerators can produce electron beams with energies of 1.75 MeV orlower. With current linear accelerator designs the electron current (andconsequently the X-ray beam flux) falls with decreasing electron energy.This can place a lower limit on the electron beam energy that it ispractical to use. In some embodiments, the control of a linearaccelerator is set to produce a lower energy electron beam 15 whenswitching to a low-Z target 50 and to increase the energy of electronbeam 15 when switching back to high-Z target 16. In some embodiments theelectron beam energy is set to a value of 3 MeV or less for generatingan imaging X-ray beam 17.

FIG. 2 is a graph illustrating CNR as a function of dose for bone andlung tissue images for three different X-ray beams. Curves 55A and 55Bare for a beam generated by impinging a 3.5 MeV electron beam on analuminum target. Curves 57A and 57B are for a 6 MV therapeutic radiationbeam. Curves 56A and 56B are for a beam generated by impinging a 7.0 MeVelectron beam on an aluminum target. It can be seen from FIG. 2 that,for the same CNR, using the 3.5 MeV electron beam with a low-Z targetcan reduce dose by a factor exceeding 7 as compared to images made usingthe 6 MV therapeutic beam. FIG. 2 is taken from Robar et al.,Megavoltage cone-beam imaging with low-Z targets Medical Physics, Vol.36, No. 9, September 2009, which is hereby incorporated herein byreference.

In some embodiments cone beam imaging is performed using an X-ray beamderived from a MeV electron beam in which the beam is shaped to conformto the beam's-eye-view profile of a volume of interest. The shape of thebeam may be controlled by a multileaf collimator on its own or incombination with adjustable jaws, for example. In some embodiments theX-ray beam is generated using a low-Z target such as an aluminum carbonor beryllium target. In some embodiments a flattening filter is notpresent during the imaging.

The techniques for imaging volumes of interest described herein are notlimited to X-ray beams generated by MV electron beams. These techniquesmay also be applied to kV on-board-imaging (OBI) systems of the typethat are becoming common on medical linear accelerators. To apply thesetechniques using an OBI system one would need to equip the OBI system(kV x-ray source) with some type of beam shaping device such as amultileaf collimator.

In some embodiments planning for imaging is performed using a treatmentplanning system. Treatment planning systems typically include functionsfor setting a beam shaper to match projected contours of a targetvolume. Such functions may be applied in planning for imaging. Thevolume of interest may be selected to provide imaging information thatis useful for guiding the delivery of radiotherapy. For example: thevolume of interest may be selected to include all or a portion of alesion to be treated as well as all or a portion of a sensitivestructure nearby the lesion that it is intended to spare. As anotherexample, the volume of interest may be selected to include a fiduciarymarker (e.g. a feature of a bone or other object that can be used as areference point for determining a position or orientation of a subject).

In some embodiments a treatment system includes data defining athree-dimensional shape of a lesion to be treated and data defining avolume of interest for imaging is created by expanding the threedimensional shape such that the imaging volume of interest includes thelesion as well as a layer of tissues immediately outside of the lesion.Radiation may be delivered in sessions over the course of several days.Some treatments may be spread out over weeks. Over such a period asubject may lose weight or gain or lose fluids. A lesion being treatedmay shrink or grow. Imaging of the volume of interest may be performedbefore each session, for example.

Obtaining images during the course of a radiation treatment can beespecially beneficial in the case where a lesion being treated is insoft tissue and may move around depending upon the subject's posture orchanges in the subject or cases where a lesion may change in shape orposition during the time span over which the treatment is delivered.

As one example application, consider the case where a tumor or otherlesion to be treated by radio therapy is close to a subject's spinalcord. It is desired to spare the spinal cord while delivering radiationto the lesion. An imaging volume of interest may include all or aportion of the lesion as well as all or a portion of the part of thespinal cord that passes near to the lesion. Images of this volume ofinterest may be co-registered with an image of a volumetric dosedistribution to be delivered by a treatment plan and used to verify thata plan to deliver radiation to the lesion will, catch the entire lesionwhile, as much as possible sparing the spinal cord. One can determinefrom the image whether the subject's position is exactly correct suchthat the therapeutic radiation will be delivered to the lesion and avoidthe spinal cord.

If the image indicates that the volumetric dose distribution to bedelivered by the treatment plan is not ideal for some other reason—forexample where the image indicates that: the lesion has grown so that aportion of the lesion would not be adequately irradiated by executingthe treatment plan; or the lesion has shrunk so that areas external tothe lesion would receive more radiation than necessary by executing thetreatment plan—then it may be necessary to establish a new treatmentplan based on new imaging.

The image can also be used to verify that the subject is positioned insuch a manner that the planned radiation dose will be delivered to thetumor. As another example application a small volume of interest mayinclude the prostate, as well as interfaces with the rectum and bladder.Shaping an X-ray cone beam to conform to such a volume of interestpermits imaging the prostate while largely sparing peripheral volumes ofthe pelvis from radiation exposure.

An advantage of imaging using a shaped beam is that the total dosedelivered during imaging is reduced since radiation dose is greatlyreduced outside of the boundary of the shaped beam. However, imagetruncation resulting from the beam shaping can result in severe imagingartifacts that can deleteriously affect the usefulness of the resultingimages. Such artifacts tend to arise particularly where a number ofimages are acquired and combined into a 3D dataset using computedtomography (CT) imaging techniques.

Some embodiments perform CT imaging using a cone beam. In suchembodiments, images are obtained for each of a plurality of differentgantry angles. For each of the images the cone beam is shaped to conformwith the projection of a volume of interest. The shaping is performedtaking into account the geometry of the cone beam. The projection is aconical projection following rays of the imaging cone beam. Since thebeam is diverging the shape imposed by a beam shaper such as a multileafcollimator is magnified as the beam propagates to the subject.Consequences of this geometry are that beam shaper apertures set in aplanning system to shape the beam to match a volume of interest need tobe scaled to take into account this magnification.

Where an imaging beam has a different beam geometry from a therapeuticbeam then this different beam geometry needs to be taken into accountboth for establishing beam shaper settings to appropriately shape theimaging beam and for scaling acquired images to provide accurate spatialcalibration.

The resulting images are then combined to provide a 3D data set. In somesuch embodiments the individual images are filled outside of theprojection of the volume of interest with image data. The image dataused for the fill may be obtained in various ways from various sourcesas discussed below. Filling the individual images prior to combiningthem to yield a 3D dataset can significantly reduce truncationartifacts.

The filled images may be combined using suitable cone-beam CT (CBCT)techniques. CBCT imaging techniques which combine 2D images frommultiple angles to provide a 3D dataset are known to those of skill inthe art. In some embodiments, a Feldkamp-Kress-Davis (FDK) filteredback-projection algorithm is applied for reconstructing images from thedataset. Optionally the images are filtered prior to back-projection.For example, Shepp-Logan, Hamming, Cosine or Hann filters may beapplied. Cone-beam CT imaging software for processing sets of imagesinto 3D datasets and reconstructing images from the datasets iscommercially available. Such software may be applied to combine thefilled individual images and to generate reconstructed images of thesubject in desired planes.

FIG. 3 is a flow chart illustrating an imaging method 60 according toone embodiment. In block 62, method 60 sets a MV radiotherapy sourceinto an imaging mode to deliver radiation for imaging. Block 62 mayinclude, for example, replacing a high-Z target with a low-Z target,removing a flattening filter and setting the electron-beam energy for alinear accelerator. In some embodiments, in the imaging mode, 35% ormore of the photons in the X-ray beam have energies in the range of 25keV to 150 keV. Block 62 is optional in the case that the radiotherapysource is already appropriately set up.

Loop 64 acquires images from a desired number of gantry angles. In block65A the gantry is positioned for the current image. In block 65B a beamshaper such as a multileaf collimator is set to shape an X-ray beam tothe shape of a projection of the volume of interest. Block 65B may, forexample comprise setting one or both of an angle of rotation of amultileaf collimator and leaf positions for leaves of the multileafcollimator. In block 65C, which is optional, beam parameters for theexposure are set. Block 65C may be useful in the case of an off-axisvolume of interest in cases where the flux of X-ray beam 17 variessignificantly across the beam. For example, when the volume of interestis located in a higher-flux portion of X-ray beam 17 electron beamcurrent may be reduced below the current used when the volume ofinterest is located in a lower flux portion of X-ray beam 17. This canreduce the dose delivered to the subject.

In block 65D the subject is exposed to the shaped X-ray beam and animage 66 is acquired. In block 65E the image 66 is corrected tocompensate for variations in the fluence of the imaging beam as well asvariations in the sensitivity of the detector used to obtain images 66.

The corrections in block 65E may be based on previously-acquiredcalibration information that characterizes the X-ray beam. For example,prior to all image acquisition, dark field (IDF) and flood field (IFF)images may be acquired for image calibration. A dark field image isobtained without applying any beam to the imaging panel. A dark fieldimage may be applied to correct for any variations of dark currentbetween individual detector elements.

A flood field image may be acquired by exposing the entire sensitivearea of the imaging panel to the beam and acquiring an image. A floodfield image can be applied to correct for non-uniformities in thefluence of the imaging beam as well as for non-uniformity in detectorresponse.

Any images acquired after IDF and IFF images may be corrected insoftware by subtracting the IDF image and dividing the result by the IFFimage to produce a corrected image. In some embodiments correction isperformed by computing the result:

$\begin{matrix}{{IM}_{f} = \frac{{IM}_{i} - {I\; D\; F}}{I\; F\; F}} & (2)\end{matrix}$

where IM_(i) is the uncorrected image data 66, IM_(f) is the correctedimage data and IDF and IFF are the dark field and light field images asdefined above.

Block 65F determines whether more images are to be obtained. Block 68fills the portions of each image 66 lying outside of the volume ofinterest with image data. Block 69 combines the filled images 66 toprovide a 3D dataset 70. Block 69 may comprise back-projection of theimages 66. Optionally block 69 comprises filtering images 66 prior toback-projecting images 66. Block 72 uses dataset 70 to construct anddisplay an image. Block 72 may comprise, for example, constructing animage for coronal, sagittal or axial slices through the volume ofinterest.

Method 60 may be varied many ways. For example, blocks 65E and 68 may beperformed inside or outside of loop 64. Where blocks 65E and 68 areperformed inside loop 64 part or all of block 69 may also be performedin loop 64.

For clarity of explanation method 60 is described above as operating ina step-and-shoot mode with discrete motions of a gantry and beam shaperbetween imaging positions. The invention is not limited tostep-and-shoot modes. The skilled reader will understand that imagingmay be performed while the gantry is rotated continuously together withconcurrent, synchronized, dynamic motion of the beam shaper. Forexample, a gantry may be driven to rotate through an arc withoutstopping while leaves of a multileaf collimator are driven to execute adynamic sequence in tandem with the gantry motion. Images may beacquired at predetermined angles throughout the gantry rotation.Acquiring each image may comprise generating an imaging X-ray beam andoperating an imaging X-ray detector.

One source of image data for filling the truncated images ispreviously-acquired CT data. It is almost always the case that a CT scanfor a subject has been obtained for use in planning treatment for thesubject prior to delivery of the treatment to the subject. In someembodiments image data for use in filling images 66 is obtained fromsuch CT scan data (e.g. imaging data 43—see FIG. 1).

FIG. 4 illustrates an example method 73 for filling images 66. In block74 imaging data 43 is processed to yield a digitally reconstructedradiograph (DRR) 75 from the point of view of the beam for the gantryangle corresponding to the current image 66. The DRR 75 is constructedbased upon the geometry of the cone beam used for imaging. Low-Z target50 may be (and for linear accelerators of the type in use in 2011usually will not be) in the same location relative to the subject ashigh-Z target 16. Where low-Z target 50 is located closer to the subjectthan high-Z target 16 the low-Z imaging beam will gave greaterdivergence than the therapeutic beam. Also, photons from the low-Ztarget 50 will have substantially different energy spectralcharacteristics compared to a therapeutic beam. Algorithms forgenerating DRRs typically include the attenuation coefficient forphotons, which depends on the incident spectrum (as well as thecharacteristics of the tissues through which the photons pass). Block 74may implement an algorithm for generating DRR images 75 that takes intoaccount these factors in order to produce DRRs 75 that closely matchacquired low-Z images.

Some embodiments trade off between the quality of DRR 75 and theprocessing to generate DRR 75. A lower quality DRR 75 computed byapplying a simplified algorithm may have quality sufficient for use infilling images 66.

DRR 75 may be pre-computed and stored in which case block 74 maycomprise retrieving the appropriate DRR 75 from a data store.

Block 76 generates a mask 77 corresponding to the projection of thevolume of interest in the current image 66. Block 76 may, for example,identify as belonging to the volume of interest all pixels of image 66having values exceeding a threshold or calculate a mask 77 from datadefining the volume of interest. In an example embodiment mask 77 hasthe form of a binary image (pixel values are either 1 or 0). Mask 77 maybe a negative of the projected volume of interest (for example, maskvalues may be set to 1 for pixels in which the pixel value from image 66is less than a threshold and 0 otherwise). In alternative embodimentsmask 77 may be a positive of the projected volume of interest.

Block 78 combines the image 66 with the corresponding DRR 75 using mask77 to yield a filled image 67 which is the same as image 66 within theprojected boundary of the volume of interest and is made up of imagedata from DRR 75 outside of the projected boundary of the volume ofinterest.

In an example embodiment mask 77 has the form of a binary image (pixelvalues are either 1 or 0). The mask 77 may be a negative of theprojected volume of interest (for example, mask values may be set to 1for pixels in which the pixel value from image 66 is less than athreshold and 0 otherwise). DRR 75 may be multiplied by mask 77 and theresult may be added to image 66 to obtain a filled image 67.

FIG. 5 illustrates an example image 66, a DRR 75 a mask 77 and a filledimage 67. In filled image 67 the boundary of the projected volume ofinterest is indicated by dashed line 79A, a region 79B inside boundary79A comprises image data from image 66, a region 79C outside boundary79A comprises image data from DRR 75.

Optionally pixel values in the image data used to fill truncated images66 are matched to pixel values in adjacent pixels within the volume ofinterest of truncated images 66. For example, where the image data usedto fill truncated images 66 includes the volume of interest, acorrelation in pixel values can be established by comparingcorresponding regions in the truncated image 66 and the DRR 75 or otherimage data being used for filling. For example, one may generate a tonemapping curve (or ‘grey level transformation’) by plotting the values ofpixels in the truncated image versus the values of corresponding pixelsin the filling image data and apply the tone mapping curve to modify thefilling image data to better match the truncated image 66. The tonemapping curve may be parameterized by fitting a parameterized curve tothe plotted curve.

Especially where images 66 are acquired for closely-spaced gantry anglesit is not mandatory to calculate a separate DRR for filling every image66. Optionally the same DRR may be reused to fill images 66 for two ormore closely-spaced gantry angles. For example a separate DRR may becomputed for the images 66 taken within a range of gantry rotationangles. The range may span 2 or 4 or 5 degrees for example or evenlarger angles such as 15 or 20 degrees.

Image data for use in filling images 66 may also be obtained by takingsome images using unshaped (full-frame) X-ray beams or X-ray beamsshaped to have boundaries outside of the projected boundary of thevolume of interest. For example, one such images may be acquired for usein filling other images within a range of gantry angles. In an exampleembodiment N images 66 are acquired. An image 66 is acquired for every mdegrees of gantry rotation over an angular range spanning (N−1)×mdegrees. For example, an image may be acquired every 2 or 3 degrees ofgantry rotation over a suitable range (e.g. a range spanning about 180degrees-180 degrees plus the angle of the X-ray cone beam is ideal).

The X-ray cone beams used to acquire images 66 may be shaped to matchthe projected boundary of a volume of interest except that every n^(th)image 66 may be a full-frame image. Each image 66 requiring fill may befilled using image data from the nearest full-frame image. In someembodiments full-frame images are only obtained for every 15 or 20degrees or more of gantry rotation. In some embodiments, n is 5 or moreor 10 or more.

In experiments done imaging a RANDO™ head phantom it was found that thequality of the portions of reconstructed images within the volume ofinterest was quite insensitive to the angular separation betweenfull-frame images used for providing image data for fill. It was foundthat image quality in the region outside the volume of interest iscompromised by sparse projection data. However, for many applicationsimage quality in the region outside the volume of interest isunimportant.

Although reducing artifacts is a main benefit of filling images 66,filling may provide some additional benefit as a result of improvementof CNR. FIG. 6A is a graph showing contrast as a function of dose for CTresults based on filled (curve 80A) and unfilled (curve 80B) images.FIG. 6B is a graph showing noise as a function of dose for CT resultsbased on filled (curve 81A) and unfilled (curve 81B) images. FIG. 6C isa graph showing CNR as a function of dose for CT results based on filled(curve 82A) and unfilled (curve 82B) images.

Other types of image reconstruction are also possible. For example,images may be reconstructed using pi-line reconstruction as described inZou Y. et al. Exact image reconstruction on PI-lines from minimum datain helical cone-beam CT Phys Med Biol. 2004 Mar. 21; 49(6):941-59 whichis hereby incorporated by reference herein. Such reconstructions may beused in some embodiments.

The data on which FIGS. 6A, 6B, and 6C are based was for a beam shapedusing a 10 cm aperture. Fill was provided from full-field images havinga width of 26 cm. One full field image was obtained for each 20 degreesof gantry rotation. Truncated images were filled using image data fromthe closest full-field image.

The imaged subject was a bone object in a uniformly cylindrical waterphantom. It can be seen that filling the images beneficially increasescontrast and decreases noise. Thus, CBCT data of a desired quality canbe obtained for at least some subjects with lower radiation doses whenCBCT images are filled than when the images are subjected to CTprocessing without being filled.

FIGS. 7A and 7B illustrate dose from an imaging exposure to cone-beamX-ray radiation as a function of position for 4 cm diameter and 8 cmdiameter cylindrical volumes of interest. The doses plotted in FIGS. 7Aand 7B were measured using thermoluminescent dosimeters inside aphantom. FIG. 7A shows dose as a function of position in ananterior-posterior direction in a saggital plane through the volume ofinterest. Curve 83A is dose for a full-field image. Curve 83B is dosefor a volume of interest 8 cm in diameter. Curve 83C is dose for avolume of interest 4 cm in diameter. FIG. 7B shows dose as a function ofposition in a left-right direction in a coronal plane through the volumeof interest. Curve 84A is dose for a full-field image. Curve 84B is dosefor a volume of interest 8 cm in diameter. Curve 84C is dose for avolume of interest 4 cm in diameter.

Repositioning the leaves of a multileaf collimator takes some time.Larger movements typically require longer times. To reduce the timerequired for obtaining full-field images and images using X-ray beamsshaped to conform to regions of interest one can acquire a number offull-field images and then acquire a number of images using shaped X-raybeams. For example, one could move the gantry through a range of anglesin one direction while obtaining full-field images and then move thegantry again through the range of motion while acquiring images usingshaped X-ray beams.

In an example embodiment, the gantry acquires images 66 using shapedX-ray beams as it is moved through about 180 degrees. Subsequently thedirection of gantry rotation is reversed and full-field images areacquired as the gantry is moved back through the angular range. In thealternative, full field images can be acquired first and images can thenbe acquired using a shaped beam. In another example embodiment thegantry is rotated through 360 degrees and the MLC is controlled to shapethe X-ray beam for images taken in a ½ rotation of the gantry and toobtain full-frame images in the other ½ rotation of the gantry.

Another option for filling images 66 is to fill image areas outside ofthe volume of interest by extrapolation from image areas inside thevolume of interest. This may be done on a line-by-line basis, forexample. A fitting function such as a polynomial function may be fit topixel values inside an image area corresponding to a volume of interest.Pixel values for image areas outside the volume of interest can then beset according to the fitting function. The fitting function may bechosen to avoid sharp discontinuity at the boundary of the part of theimage corresponding to the volume of interest. The fitting function maybe a lower order polynomial function for example.

As a simpler alternative to shaping the X-ray beam to conform with avolume of interest, truncated images may be obtained by shaping theX-ray beam with a predetermined on-axis shape that is the same for allapertures (i.e. the same for each image 66). The shape could, forexample, be a circle, ellipse, oval or other rounded shape, a stripe orrectangle or the like.

An advantage of VOI CBCT image acquisition is that radiation dose isreduced not only outside of the volume of interest (i.e. inlargely-shielded patient volumes) but also within the volume of interestitself. This advantage arises where a beam is shaped using aperturesthat are small relative to full-field acquisition. In such cases thecontribution to the dose delivered to the volume of interest by photonsscattered from outside of the volume of interest is reduced. This effectof beam shaping is especially significant for low-Z imaging beamsbecause in such X-ray beams (which have lower effective beam energiesthan beams generated from a high-Z target), the proportion of scatteredphotons tends to be higher relative to primary photons. The reduction ofscattered photons can improve aspects of image quality as compared tofull field imaging since scatter degrades both contrast and spatialresolution.

It can be appreciated that some embodiments of the imaging apparatusdescribed above offer the advantage that imaging can be performed in amanner that is tightly integrated with the delivery of therapeuticradiation. No auxiliary imaging system is required. Associated overheadin terms of cost and quality assurance are reduced. A further advantageoffered by some embodiments is that the imaging and therapeutic beamsare coaxial and so a beam's-eye-view image for the imaging beam is alsoa beam's-eye-view image from the perspective of the therapeutic beam.

In some embodiments, imaging as described herein may be performedsimultaneously for a plurality of different volumes of interest. Thedifferent volumes of interest may be disconnected from one another ormay be contiguous or even overlap.

Depending upon the capabilities of a beam shaper (e.g. a MLC) it may bepossible to obtain images 66 using beams that are shaped to expose aplurality of volumes of interest while reducing or substantiallyeliminating exposure to radiation outside of the regions of interest.For example, FIG. 8A shows a beam's-eye view of a plurality of volumesof interest 86A, 86B, and 86C projected into the plane of a multileafcollimator and shows positions of leaves 88 of the multileaf collimatorthat would shape an X-ray cone beam to expose the volumes of interest.Where the plural volumes of interest have sizes and locations such thata multileaf collimator can be controlled to shape the X-ray beam toexpose the plural volumes of interest then imaging the plural volumes ofinterest may be performed as described above.

In cases where the multileaf collimator or other beam shaper cannot becontrolled to shape the X-ray beam appropriately then the plural volumesof interest may be imaged using a plurality of apertures. FIG. 8B is anexample of an arrangement of volumes of interest 87A, 87B, and 87C(collectively volumes 87) viewed from an angle for which it isimpossible to use a multileaf collimator to shape an X-ray beam to matchthe projection of the volumes of interest. This is because to blockradiation from central area 89, at least some leaves 88 of the multileafcollimator would need to also block radiation from reaching one or moreof the volumes of interest 87.

In such cases images 66 can be acquired using two or more X-ray beamshapes and then combining the resulting images. For example, FIGS. 8Cand 8D illustrate two different configurations of the leaves 88 of amultileaf collimator. Two images obtained using X-ray beams shaped bythese configurations will image the volumes of interest 87 but, each ofthe X-ray beams is shaped to avoid exposure outside of volumes ofinterest 87.

In some embodiments a multileaf collimator is rotated about its own axisto allow leaves 88 to be adjusted to better match the contours of theboundaries of the projections of volumes of interest.

FIG. 9 illustrates a method 90 for obtaining 3D image data coveringmultiple volumes of interest. Loop 92 is repeated for a plurality ofgantry angles that are spaced apart by a suitable angular distance.Block 94 determines whether the beam shaper can be configured to shapethe X-ray beam to conform with the projection of the volumes of interestfor the current angle. If so (YES result), block 95 configures the beamshaper to shape the X-ray beam and block 96 acquires an image 66.

If block 94 determines that it is not possible to configure the beamshaper to shape the X-ray beam to conform with the projection of thevolumes of interest for the current angle (NO result) then block 97determines a plurality of beam shaper configurations that each shape theX-ray beam to expose a portion of the projections of volumes ofinterest. In aggregate, the volumes of interest are all exposed byexposures using made using the plural beam shaper configurations andradiation is blocked from other areas. Block 98 configures the beamshaper to shape the X-ray beam according to a current one of theconfigurations and block 99 acquires an image 66A. Loop 100 is repeateduntil images 66A have been obtained using X-ray beams shaped by each ofthe configurations determined by block 97.

Block 102 combines images 66A into an image 66. Block 102 may comprise,for example, setting all pixel values in images 66A that are below athreshold value to zero and then summing the images 66A.

Block 106 fills images 66 as described above. Block 108 processes thefilled images 67 to provide a 3D data structure. Block 110 recreates anddisplays an image in a plane passing through one or more of the volumesof interest based on the 3D data structure. Block 110 optionallydisplays highlighting, lines or other indicia indicating boundaries ofthe volumes of interest on the displayed images.

It is not necessary that all volumes of interest be imaged in the sameimage quality. Some volumes of interest may be imaged using higher dosesthan other volumes of interest. For example, a particular volume ofinterest (e.g. a target volume for radiotherapy and its immediatemargin) may be imaged with a dose sufficient to provide a relativelyhigh contrast-to-noise ratio while simultaneously capturing the externalsurface of the patient at lower CNR.

Imaging different volumes of interest with different doses may beachieved in various ways. One approach is to image different volumes ofinterest using different apertures (different beam shaping). Exposure ineach aperture may then be controlled to achieve a desired image qualityin the volume(s) of interest corresponding to the aperture. Anotherapproach is to image a second volume of interest in fewer apertures thana first volume of interest. This may be achieved, for example, byshaping an imaging beam to image the first volume of interest for anumber of steps of gantry rotation and opening the aperture to includethe second volume of interest for only some of the gantry rotationangles. For example, a full-field exposure could be taken at everyn^(th) gantry rotation step while exposures at other gantry rotationsteps may be limited by shaping the beam to conform with the projectionof the first volume of interest. Such full-field exposures may also beused as a source of fill image data as described above. These twoapproaches may be combined.

Apparatus and methods according to some embodiments calculate volumetricradiation doses delivered during imaging. The imaging radiation dosesmay be included in radiation dose estimates being used by a treatmentplanning system. In some embodiments a treatment planning system isconfigured to optimize a radiation treatment plan based upon doseestimates that include radiation dose delivered during imaging. Suchembodiments may but do not necessarily apply the imaging methods asdescribed above. In some embodiments doses from other imaging modes(e.g. MV CBCT) may be included in dose estimates used in optimizing atreatment plan.

Calculating dose delivered by imaging or therapeutic beams typicallyrequires knowledge of the location of the external surface of thesubject (e.g. the location of the subject's skin surface) since the dosecalculation should take into account attenuation of the beam with depth.In some embodiments where it is desired to estimate an imaging dosebased on VOI CBCT images, or to recalculate a dose of therapeuticradiation based on VOI CBCT images, a volume of interest that includesthe subject's skin surface may be imaged using a relatively low dose. Itmay be sufficient to have image quality just good enough to determinethe location of the external surface of the subject. For example, avolume of interest centered on a target volume can image the targetvolume at high quality while an outer volume that includes the externalsurface of the subject can be imaged at lower quality and dose asdescribed above.

In some embodiments, the external region including the subject'sexternal surface is post processed to reduce noise. Image details foraccurate dose estimation may be sourced from planning CT data and fittedto the external region by deformable co-registration.

In embodiments where imaging is not performed using the same beam usedfor radiation therapy then the treatment planning system andradiotherapy apparatus may be commissioned and validated for both theradiotherapy beam and the imaging beam. For example, apparatus may becommissioned for both a treatment beam generated using a high-Z targetflattened using a flattening filter and an imaging beam generated usinga low-Z target and no flattening filter. The commissioning may reflectdifferences in the geometries of the imaging and treatment beams as wellas differences in the spectral makeup of the imaging and treatmentbeams.

FIG. 10 is a block diagram of a radiotherapy system 120 that includes atreatment planning unit 112 operating in conjunction with a radiationdelivery machine 122. Radiation delivery machine 122 may comprise, forexample, a linear accelerator.

Radiation delivery machine 122 comprises interchangeable targets 124Aand 124B. A controller 125 is connected to configure radiation deliverymachine 122 to deliver a therapy beam using target 124A or an imagingbeam using target 124B.

Treatment planning unit 112 has access to a data store 127 containingdata 128A and 128B that respectively characterize the therapy beam andthe imaging beam. Treatment planning unit 112 has access to a data store129 (which may be part of data store 127 or separate from data store127) containing 3D imaging data 130 for a subject. 3D imaging data 130may include data acquired from one or more of CT scanning, magneticresonance imaging (MRI), positron emission tomography (PET), ultrasoundscans or other imaging modalities.

An operator can work with treatment planning unit 112 to establish atarget radiation dose distribution. For example, the desired target dosedistribution may be essentially constant inside a tumor in the subjectand zero (or as close to zero as possible) in normal tissues surroundingthe tumor. The operator may view images generated from images 3D imagingdata 130 for assistance in specifying the target dose distribution. Inthe illustrated embodiment target dose distribution data 132 specifiesthe target dose distribution calculated by treatment planning unit 112.

The operator can work with treatment planning unit 112 to establish atreatment plan that attempts to efficiently and accurately deliver thetarget radiation dose distribution as specified by target dosedistribution data 132. The treatment plan comprises instructions 134that can be executed by controller 125 of radiation delivery machine 122to deliver radiation to the subject. Instructions 134 may comprise, forexample instructions identifying gantry angles, beam shaper settings(e.g. leaf positions and rotation angles for a multileaf collimator, jawpositions, etc.) and beam conditions (e.g. accelerator energy andfluence). The instructions may specify a step-and-shoot mode ofradiation delivery and/or dynamic modes of radiation delivery.

A treatment plan may provide for delivery of the radiation in a numberof fractions. The fractions may be delivered at intervals (for exampleone fraction per day, one fraction every few days or one fraction everyseveral hours). The interval between fractions typically depends uponthe condition being treated and the treatment approach decided upon bythe managing physician in consultation with the subject. Each fractionmay comprise irradiation from a number of gantry angles with radiationthat is shaped in one or more ways at each gantry angle. Some treatmentplans involve many fractions and are designed to be executed over aperiod of days or weeks. Other treatment plans are designed to beexecuted over shorter periods. Some treatments, such as certainradiosurgery treatments can be executed by delivering a single fraction.

Various approaches to treatment planning are known to those of skill inthe art. Treatment planning systems for radiation therapy arecommercially available. One example is the ECLIPSE™ treatment planningsystem available from Varian Medical Systems of Palo Alto, Calif.Treatment planning unit 112 may comprise an add-on to an existingtreatment planning system or a stand-alone treatment planning system,for example.

Treatment planning unit 112 is configured to develop one or more imagingsequences in conjunction with a treatment plan. The imaging sequencesmay, for example, be used to verify that the subject is properlypositioned for the delivery of each fraction. It is necessary to ensurethat the subject is in the proper position relative to radiationdelivery machine 122 for each fraction. In an example embodiment, atreatment plan may include an imaging sequence at the beginning of eachfraction. After one or more volumes of interest have been identified theimaging sequence for imaging those volumes of interest may beautomatically generated and added to the treatment plan to be executedbefore each fraction or before certain fractions.

The imaging sequence may specify acquisition of images over a fullgantry rotation, gantry rotation of about 180 degrees (e.g. 180 degreesplus the angle of the X-ray cone beam). Other angular ranges may also beused depending on the application. For example, methods and apparatus asdescribed herein may be applied in ‘tomosynthesis’, a technique in whicha narrow rotational range is selected in order to reconstruct an imagein a desired, single plane through the subject. The small range ofangles is chosen based on the desired image plane. For example, onecould acquire data to reconstruct just the sagittal or just the coronalplane of the subject by acquiring projections in a narrow range aroundthat plane. Methods and apparatus as described herein may be applied,for example to provide a low-Z VOI tomosynthesis.

In some embodiments the imaging sequences comprise instructions that canbe executed by controller 125 of radiation delivery machine 122 to placeradiation delivery machine 122 in an imaging mode, deliver imagingradiation to the subject, and trigger an imaging detector to collectimage data for each image. The instructions may comprise, for exampleinstructions identifying gantry angles, beam shaper settings (e.g. leafpositions and rotation angles for a multileaf collimator, jaw positions,etc.) and beam conditions (e.g. accelerator energy and fluence). In someembodiments the instructions specify beam shapes for the acquisition ofimages for one or more volumes of interest as described above.

In some embodiments the instructions specify different beams for imagingand treatment. The beams may differ, for example in terms of energyspectra and/or geometry. In some embodiments, imaging beams andtreatment beams are both generated using a MV electron beam from thesame linear accelerator. FIG. 10 shows imaging instructions 136. Imaginginstructions 136 may be combined with or separate from treatmentinstructions 134.

Treatment planning unit 112 may be configured to estimate a 3D imagingdose distribution 140 that will be delivered to the subject uponexecution of the imaging sequence. 3D imaging dose distribution 140 maybe used in the optimization or re-optimization of a treatment plan.

FIG. 11 illustrates one method 150 for using a 3D imaging dosedistribution in the optimization of a treatment plan. In block 152 atreatment plan is initialized. In block 154 the dose distribution thatwould be delivered by the initial treatment plan is calculated usingdata 128A that characterizes a therapeutic beam to be used in executingthe treatment plan. In block 155 the dose distribution of block 154 iscompared to the target dose distribution 132.

Method 150 includes an optimization loop 156. In block 156A thetreatment plan is modified. The modification may be stochastic ordetermined according to another optimization methodology.

In block 156B the dose distribution that would be delivered by themodified treatment plan is calculated using data 128A. In block 156C thedose distribution of block 156B is compared to the target dosedistribution 132. Block 156D determines whether the treatment plan asmodified in block 156A is better than the treatment plan prior tomodification by block 156A (e.g. the modified treatment plan satisfiesrelevant criteria and a the comparison of block 156C indicates a dosedistribution that is closer to target dose distribution 132). Block 156Dkeeps the better of the modified treatment plan and the treatment planprior to modification by block 156A. Block 156E determines whether atermination condition is satisfied. If so (YES result) optimization loop156 ends. Otherwise (NO result) processing continues at block 156A. Inblock 158 the optimized treatment plan is stored.

Method 150 includes block 160 that calculates 3D imaging dosedistribution 140 from previously established imaging instructions 136and data 128B characterizing an imaging beam. Blocks 154 and 156C addall or parts of imaging dose distribution 140 to the dose distributionsestimated for the treatment plan.

A wide variety of computer-implemented treatment planning algorithms areknown and described in the patent and technical literature. It istypical that such algorithms include optimization steps in which a dosedistribution is estimated and, based upon the estimated dosedistribution (usually based on a comparison of the estimated dosedistribution to a target dose distribution), further optimization stepsare performed. Many such algorithms are inverse planning algorithmswhich start with a desired radiation dose distribution and attempt toestablish a treatment plan (set of instructions for a radiation deliverysystem) that will deliver the desired radiation dose distribution to thesubject.

One non-limiting aspect of the present invention is to provide newcomputer-implemented treatment planning algorithms and systems bymodifying such existing algorithms by: determining an imaging radiationdose, as described herein and including that imaging radiation dose inthe estimated dose distributions used by the treatment planningalgorithm—thereby arriving at a treatment plan in which the imaging doseis counted as contributing to the therapeutic dose and the treatmentplan is optimized taking into account a dose expected from imagingduring delivery of the treatment.

Some embodiments provide an automated treatment planning system thatreceives an imaging volume from a user. The imaging volume may, forexample, be defined relative to previously-obtained imaging data for thesubject. The previously-obtained imaging data may comprise data from aCT scan, MRI or other imaging modality or modalities, for example. Thetreatment planning system also receives from a user informationspecifying an imaging frequency (e.g. once per fraction, once per day,once every two days or the like). Optionally the treatment planningsystem receives from a user information specifying a required imagingquality. Based upon the user-supplied information and datacharacterizing an imaging beam (e.g. a low-Z beam as described above orother imaging beam) the treatment planning system calculates an imagingdose distribution. The treatment planning system receives from the userdefinition of a target radiation dose distribution and then applies aninverse planning algorithm to generate a treatment plan for delivering aradiation dose distribution that is as close to the target radiationdose distribution as practical. In the inverse planning algorithm thepreviously-calculated imaging dose is used as a baseline dose.

In some embodiments the incorporation of imaging dose distribution 140in the dose estimates used in optimizing a radiation treatment planprovides one or more of the following advantages: more accurateestimation of the dose that will be delivered to a subject uponexecution of a radiation treatment plan with associated imaging; theopportunity to obtain higher quality images by increasing imaging doseswithout increasing the overall dose delivered to the subject (since anincrease in imaging dose at a location can be compensated for bymodifying the treatment plan to decrease the therapeutic dose deliveredat that location), and the opportunity to leverage the technology inexisting treatment planning systems (treatment planning systemstypically include functions for estimating dose distributions andfunctions for matching beam shapes to target regions, these functionscan be modified relatively easily for estimating imaging dose andplanning imaging beams).

At imaging machine 122 a subject may be placed in position and imaginginstructions 136 may be executed to obtain image data 70. Image data 70may comprise a VOI CBCT image set, for example. Imaging machine 122 mayperform automated co-registration between image data 70 and a targetdose distribution. Imaging machine 122 may display alignment indiciaindicating where features in image data 70 ought to be located when thesubject is properly positioned for treatment. A user may view the imagesand alignment indicia to determine whether it is necessary to repositionthe subject and, if so, to determine repositioning parameters.

In addition or in the alternative, image processing may be performed onimage data 70 to locate fiducial features and to compare locations ofthose fiducial features to target locations. Non-limiting examples offiducial features are gold seeds that have been implanted in the subjectat known locations relative to a target volume; a tumor of a type thatcan be imaged with sufficient contrast to be detected; features of bonesand the like. Radiation delivery machine 122 may include an imagingcontrol that permits the user to view images of a target volume invarious planes and/or from various viewpoints to check that surroundingtissues are not receiving more radiation dose than necessary.

Since imaging and therapeutic radiation are both delivered by radiationdelivery machine 122 therapy can be done in the course of acquiring animage set or vice versa. This can allow, for example, images to beacquired as therapy proceeds. In some embodiments, images acquired inthe course of delivering therapeutic radiation are 2-D images taken in abeam's eye view direction. Such images may be obtained by switchingradiation delivery machine 122 into an imaging mode, obtaining an image,and switching radiation delivery machine 122 back into a therapy modewithout changing the gantry angle.

In some embodiments a treatment planning system is configured toautomatically output instructions for acquiring 2-D images at one ormore times during delivery of a fraction. The frequency of imaging maybe selectable. Upon execution of the treatment plan the 2-D images maybe acquired and displayed on a monitor associated with a radiationdelivery machine (e.g. a linear accelerator). An operator viewing theimages can verify that the treatment being delivered appears to bedelivering the radiation to the desired target volume.

The images may be co-registered with indicia indicating desiredalignment of features in the images and/or a representation of thedistribution of dose being delivered by the therapeutic radiation (or acombination of the doses from therapeutic radiation and imagingradiation). A user such as a radiation technician or a physician viewingthe images can check to ensure that the radiation is being deliveredaccording to plan.

Certain implementations of the invention comprise computer processorswhich execute software instructions which cause the processors toperform a method of the invention. For example, one or more processorsin a treatment planning system or radiation delivery machine mayimplement the methods of FIGS. 3, 4, 5, 9 and 11 or other methodsdescribed above by executing software instructions in a program memoryaccessible to the processors. The invention may also be provided in theform of a program product. The program product may comprise any mediumwhich carries a set of computer-readable signals comprising instructionswhich, when executed by a data processor, cause the data processor toexecute a method of the invention. Program products according to theinvention may be in any of a wide variety of forms. The program productmay comprise, for example, physical media such as magnetic data storagemedia including floppy diskettes, hard disk drives, optical data storagemedia including CD ROMs, DVDs, electronic data storage media includingROMs, flash RAM, or the like. The computer-readable signals on theprogram product may optionally be compressed or encrypted.

Where a component (e.g. a software module, processor, assembly, device,circuit, etc.) is referred to above, unless otherwise indicated,reference to that component (including a reference to a “means”) shouldbe interpreted as including as equivalents of that component anycomponent which performs the function of the described component (i.e.,that is functionally equivalent), including components which are notstructurally equivalent to the disclosed structure which performs thefunction in the illustrated exemplary embodiments of the invention.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. For example:

-   -   The foregoing discussion has described radiation delivery        machines of the type in which beam angle is set by rotating a        gantry. Other mechanisms may be used to change the beam angle.        The term ‘gantry angle’ is used to describe the angle from which        an imaging or therapeutic beam is incident on a subject and does        not require a gantry or other specific mechanism be used to set        that angle.    -   The invention is not limited to applications where therapeutic        radiation is delivered in the form of X-rays. Imaging techniques        and apparatus according to at least some embodiments may be        applied in cases where the therapeutic radiation comprises an        electron beam or other particle beam, for example.        Accordingly, the scope of the invention is to be construed in        accordance with the substance defined by the following claims.

1. A method for imaging comprising: generating a cone X-ray beam bydirecting a megavolt electron beam at a low-atomic-number target;shaping the cone X-ray beam to match a shape of a volume of interest ina subject; and detecting X-rays of the cone X-ray beam that have passedthrough the subject at an imaging X-ray detector.
 2. A method accordingto claim 1 wherein shaping the cone X-ray beam comprises adjustingpositions of leaves of a multi-leaf collimator.
 3. A method according toclaim 1 wherein the low-atomic-number target comprises a target ofaluminum, carbon or beryllium.
 4. A method according to claim 1 whereinthe imaging is performed in combination with delivering radiation to thesubject for radiotherapy and the method comprises switching from ahigh-atomic-number target used for radiotherapy to the low-atomic-numbertarget.
 5. A method according to claim 1 wherein the low-atomic-numbertarget has a thickness sufficient to block passage of substantially allelectrons in the electron beam.
 6. A method according to claim 5 whereinthe low-atomic-number target has a thickness of 3 mm or more.
 7. Amethod according to claim 2 wherein the low-atomic-number target issupported on a gantry that is rotatable relative to the subject and themethod comprises repeating: shaping the cone X-ray beam to match a shapeof a volume of interest in a subject; and detecting X-rays of the coneX-ray beam that have passed through the subject at the imaging X-raydetector; to obtain a plurality of images for a corresponding pluralityof different angles of the gantry relative to the subject.
 8. A methodaccording to claim 7 comprising processing the plurality of images toprovide one or more computed tomography images for at least in a planepassing through the volume of interest.
 9. A method according to claim 7wherein the volume of interest comprises a first volume of interest of aplurality of volumes of interest and the method comprises controllingthe positions of the leaves of the multileaf collimator for each of thegantry angles to shape the cone X-ray beam to match the plurality ofvolumes of interest as viewed from a source of the cone beam.
 10. Amethod according to claim 2 wherein the volume of interest is a firstvolume of interest of a plurality of volumes of interest and the methodcomprises controlling the positions of the leaves of the multileafcollimator to shape the cone X-ray beam to match the plurality ofvolumes of interest.
 11. A method according to claim 1 wherein thevolume of interest is a first volume of interest and the method furthercomprises: shaping the cone X-ray beam to match a shape of a secondvolume of interest at least partially nested within the first volume ofinterest; and detecting at the imaging X-ray detector X-rays of the coneX-ray beam shaped to match the shape of the second volume of interestthat have passed through the subject.
 12. A method according to claim 1comprising processing the images by: truncating the image at a boundaryof the volume of interest projected onto the imaging X-ray detector; andfilling the truncated image outside of the boundary of the volume ofinterest with a fill image of an area surrounding the volume ofinterest.
 13. A method according to claim 12 wherein the fill imagecomprises a digitally reconstructed radiograph of the subject.
 14. Amethod according to claim 12 comprising acquiring the fill image byrelaxing the shaping of the cone X-ray beam and detecting at the imagingX-ray detector X-rays of the cone X-ray beam with relaxed shaping thathave passed through the subject.
 15. A method according to claim 12wherein relaxing the shaping of the cone X-ray beam comprises providinga full-field cone X-ray beam.
 16. A method according to claim 1 whereinthe imaging X-ray detector comprises an electronic portal imagingdevice.
 17. A method for planning a radiation treatment for delivery bya radiotherapy apparatus comprising a radiation source that is rotatableto different beam angles around a subject and a beam shaper configuredto control a shape of a radiation beam emitted by the radiation source,the method comprising: defining at least one set of imaging conditions,each set of imaging conditions comprising at least a beam angle and abeam shape for exposing at least one volume of interest to radiation;estimating a volumetric radiation dose for the at least one set ofimaging conditions; establishing a plan for a therapeutic radiationtreatment, the plan comprising apertures for a plurality of beam angles,establishing the plan comprising optimizing the apertures to deliver adesired radiation dose to a target region of a subject while maintainingradiation dose to tissues outside of the target region below one or morethresholds; wherein establishing the plan comprises taking into accountthe estimated volumetric radiation dose for the at least one set ofimaging conditions at least in a selected region outside of the targetregion.
 18. A method according to claim 17 wherein optimizing theapertures comprises estimating volumetric radiation doses for theapertures and summing the volumetric radiation doses for the aperturestogether with the estimated volumetric radiation dose for the at leastone set of imaging conditions.
 19. A method according to claim 18wherein the selected region outside of the target region corresponds toa sensitive tissue desired to be spared by the radiation treatment andthe optimization comprises applying a cost function that valuesminimizing dose to the selected region.
 20. A method according to claim19 comprising counting radiation to be delivered by the imagingconditions toward the desired radiation dose to the target region
 21. Amethod for planning a radiation treatment, the method comprising:planning exposures of a subject to radiation to be used for imaging;computing a contribution to dose from the imaging exposures; using theimaging dose contributions in generating a treatment plan.
 22. Animaging method comprising: for each of a plurality of different beamangles: controlling a beam shaper to shape a radiation beam such thatdelivery of radiation is primarily limited to paths that pass through aplurality of volumes of interest within a subject; obtaining images ofradiation that has passed through the volumes of interest; and,processing the images to obtain volumetric images of the plurality ofvolumes of interest.
 23. A method according to claim 22 wherein the beamshaper comprises a multileaf collimator and controlling the beam shapercomprises setting positions of leaves of the multileaf collimator.
 24. Amethod according to claim 23 wherein the plurality of volumes ofinterest comprises at least three distinct volumes of interest.
 25. Amethod according to claim 22 comprising establishing the volumes ofinterest based on a previously acquired computed tomography image of thesubject.
 26. A method according to claim 25 wherein the volumes ofinterest correspond to fiducial areas within the subject.
 27. A methodaccording to claim 25 comprising processing the volumetric images of theplurality of volumes of interest to register a current orientation ofthe subject to a coordinate system.
 28. A method according to claim 27comprising modifying a radiation treatment plan based on the currentorientation of the subject and delivering radiation to the subjectaccording to the modified radiation treatment plan.